Inkjet recording is a technique in which ink droplets are ejected (jetted) onto a recording medium to form recording dots as printing. This technique does not require master plates and is advantageously used in small-quantity wide-variety printing applications. Known inkjet inks for use in the inkjet recording technique include cationically-curable inks and radically-curable inks.
The radically-curable inks have been widely used typically because these inks have rapid curability and can be derived from a wide variety of monomers. However, from the viewpoint of dischargeability, the inkjet recording technique employs low-viscosity inks, which allow oxygen in the air to readily diffuse/migrate into the inks. In addition, such inks, as being formed into small droplets upon printing, have large surface areas and are readily exposed to oxygen. Disadvantageously, therefore, the radically-curable inks significantly suffer from cure inhibition by oxygen and are inhibited from curing by oxygen, to bleed and/or to include large amounts of residual unreacted monomers which cause odors. Additionally disadvantageously, the radically-curable inks have poor substrate adhesion and require processing on the substrate surface so as to offer better adhesion.
In contrast, the cationically-curable inks do not suffer from cure inhibition by oxygen. In addition, these inks are known to have superior substrate adhesion as compared with the radically-curable inks. Patent Literature (PTL) 1, PTL 2, and PTL 3 each describe that a cationically-curable ink including a dispersion medium and a pigment can form an ink layer having excellent curability and excellent adhesion to the substrate (substrate adhesion), when monomers constituting the dispersion medium contain a vinyl ether compound having a cyclic ether skeleton in an amount of 30 weight percent or more of all monomers contained in the dispersion medium.